1. Field of the Invention
This invention relates to autofocusing in a system in which a specimen such as blood or urine suitably stained is introduced to a flow cell to form a flat, sheathed flow within the cell, the sheathed flow zone is irradiated with strobe light, and a cell image obtained by a video camera is analyzed by application of image processing. More particularly, the invention relates to a flow cell mechanism in a flow imaging cytometer in which a slender filament is passed through a sampling nozzle within the flow cell, the state of focus of an image of the filament is monitored at all times by the apparatus itself and the position of the flow cell or lens is moved finely in automatic fashion when the image of the filament is out of focus, thereby making it possible to readjust focus automatically.
2. Description of the Prior Art
In an image processing system for imaging cells which flow through a flow cell and applying prescribed image processing, an image in which the cells appear at rest by illuminating them with strobe light is obtained. Such an image, which is obtained frame by frame (i.e., every 1/30 of a second), differs from that of the immediately preceding frame. In a still camera or domestic video camera, there is almost no movement of the image from one frame to the next, and therefore focusing of the type in which camera lens is moved in fine increments is comparatively easy to perform. However, in a case where the image changes every frame, focus cannot be adjusted by comparing identical images with each other while subjecting the flow cell or lens to fine movement. Even if a focal adjustment in such case is performed manually while viewing the image, it is difficult to judge whether focus is perfect and it takes 5 minutes or more to adjust. Moreover, since the optical system can experience some slippage owing to changes in ambient temperature, re-focusing is required often.
In order to prevent defocusing due to changes in temperature, the apparatus should be controlled so as to hold the overall optical system at a constant temperature. However, such control means raises the cost of the apparatus.
A different expedient is to adopt a method in which the user measures a control fluid, which is employed in order to control the accuracy of the apparatus, after which the apparatus itself performs focusing while finely moving the flow cell or lens based upon the images of particles whose sizes are uniform within the control fluid. However, in order to correctly obtain an evaluation value indicative of whether or not focusing has been achieved in a case where only a small number of cells appear in a single imaged frame, it is necessary to derive the evaluation value from data indicative of imaged frames consisting of several dozen frames. Consequently, considerable time is required to perform an adjustment while comparing evaluation values from point to point during the fine movement of the flow cell or lens. In addition, this method necessitates not only the reagent but also labor on the part of the user, who is required to measure the control fluid.